(Plus)-Tetrahydropapaveroline, its optical isomers, and related compounds were studied for their binding to beta-adrenergic, dopaminergic, and alpha-adrenergic receptors from rat cerebral cortex. The related compounds were (plus)-trimetoquinol; (plus or minus)-, (S)-(-), and (R)-(plus)-N-norreticuline, papaveroline, 6,7-dihydroxy-l-(4-hydroxy-3-methoxybenzyl)-1,2,3,4-tetrahydroisoquinoline-l-car boxylic acid and 6,7,dihydroxy-1-(3,4-dihydroxybenzyl)-1,2,3,4-tetrahydroisoquinoline-1-carboxyli c acid, 6,7-dihydroxy-.-(4-hydroxy-3-methoxybenzyl)-1,2,3,4-tetrahydroisoquinoline,1-(3, 4-dihydroxybenzyl)-7-hydroxy-6-methoxy-1,2,3, 4-tetrahydroisoquinoline, 6,7-dihydroxy-l-(6-bromo-3,4-dihydroxybenzyl)-1,2,3,4-tetrahydroisoquinoline, norlaudanosine; 2,3,9,10-tetraacetoxydibenzo(bg)-pyrrocoline, 2,3,8,9-tetrahydroxy-isopavinan and 2,3,8,9-tetrahydroxypavinan. THP has been considered a "mammalian alkaloid"; its endogenous formation in mammals prompted us to examine its interactions with mammalian receptors and to examine compounds related to THP. (Plus)-Trimetoquinol was among the most potent of these compounds in inhibiting binding to beta-adrenergic receptors, and 6'Br-THP was more potent than dopamine in the alpha-adrenergic assay and equipotent to it in the dopaminergic receptor assay.